1. Field of the Invention
This invention relates to a multi-piece solid golf ball having improved spin, feel, durability and distance.
2. Prior Art
Solid golf balls having multilayer structure covers are well known in the art as disclosed in JP-B 48473/1992, JP-A 343718/1994, 24084/1995, 24085/1995, and 10357/1997. JP-B 8301/1995 discloses a solid golf ball having a two-layer cover having high flexural rigidity, both the inner and outer layers having high hardnesses. Since the cover outer layer is relatively thin, this golf ball has problems with respect to the cut durability when half topped and the durability against repetitive shots.
JP-A 244174/1992 discloses a golf ball comprising a solid core, an intermediate layer and a cover wherein the intermediate layer is relatively hard and thick while the cover is made soft and thin. When hit, this golf ball receives an increased spin rate and thus travels a skying trajectory, failing to extend distance.
Attempts have also been made to improve the feel and spin controllability of multi-piece solid golf balls. For example, JP-A 24084/1995 discloses a golf ball comprising a cover including a relatively hard, thick outer layer and a relatively thick inner layer. JP-A 10358/1997 discloses a golf ball comprising a cover including a soft inner layer and an outer layer slightly softer than the inner layer.
Despite these prior efforts to improve golf balls, few of the golf balls having a cover consisting of a soft thick inner layer and a relatively thick outer layer slightly softer than the inner layer are satisfactory in feel on all shots ranging from the driver to the putter. Also desired is an improvement in spin control to comply with different clubs.
When a golf ball undergoes a great deformation as on full shots with a driver, the hardnesses of the core and enclosing components have combined effects on the flight distance and feel of the ball. When a golf ball undergoes a small deformation as on short putts and approach shots, the hardness of the cover outer layer presenting the ball surface and the hardness of the cover inner layer have an influence on the control and feel of the ball.
Therefore, if the cover inner layer is soft and relatively thick as often found in prior art golf balls having a two-layer structure cover, the ball tends to receive too much spin on iron shots, inviting skying when hit into the wind. The spin increases probably because the stress reacting the impact force in a spin direction (tangential to the ball) increases due to the thickness of the inner layer. It was found that this tendency becomes outstanding on iron shots which apply a greater force in a tangential direction upon impact. These problems of skying and distance shortage must be solved before multi-piece solid golf balls can be more practically acceptable.
Therefore, an object of the invention is to provide a multi-piece solid golf ball which has sufficient spin properties to comply with different clubs and is improved in durability, feel, and distance.
The invention is directed to a multi-piece solid golf ball comprising a single solid core and a cover of two-layer structure consisting of an inner layer surrounding the single solid core and an outer layer surrounding the inner layer. In order to improve the spin, feel, durability and distance of the ball upon full shots with a driver and approach shots, we made investigations on the hardness and thickness of the cover layers and the hardness of the solid core. The inventors have found that by limiting the deflection of the solid core under a load of 100 kg to a specific range and optimizing the hardness and thickness of the cover inner and outer layers and the ratio in thickness of the outer layer to the inner layer, the ball is improved such that it may receive an appropriately increased spin rate upon approach shots. This leads to improved control, and a relatively low spin rate upon driver shots, which leads to an increased distance. Differently stated, the spin properties of the ball comply with different clubs. Additionally, the ball is fully durable against iron shots and presents a pleasant feel when hit.
To further improve the performance of this multi-piece solid golf ball, optimizing the outer diameter of the solid core is effective.
According to the invention, there is provided a multi-piece solid golf ball comprising a single solid core and a cover of two-layer structure consisting of an inner layer and an outer layer. The solid core has a deflection of 3 to 7 mm under an applied load of 100 kg. The cover inner layer has a Shore D hardness of 25 to 58 and a thickness of 0.5 to 1.4 mm. The cover outer layer has a Shore D hardness of 30 to 62 and a thickness of 1.2 to 2.3 mm. The ratio of the thickness of the cover outer layer to the thickness of the cover inner layer is from 1.1/1 to 4.6/1.
In one preferred embodiment, the solid core has a deflection of 3.0 to 6.5 mm under an applied load of 100 kg, the cover inner layer has a Shore D hardness of 25 to 55 and a thickness of 0.5 to 1.1 mm, the cover outer layer has a Shore D hardness of 30 to 60 and a thickness of 1.2 to 2.3 mm, the ratio of the thickness of the cover outer layer to the thickness of the cover inner layer is from 1.1/1 to 3.5/1, and the ball has a deflection of 3.0 to 6.7 mm under an applied load of 100 kg.